Computed tomographic (CT) scanning is the imaging modality of choice for traumatic brain injury because of its widespread availability, the rapid imaging time, the low associated costs, and its safety. CT scanning measures the density of tissues using x-rays. To standardize the imaging procedure, 5-mm slices should be obtained from the foramen magnum to the sella and 10-mm slices should be obtained above the sella, parallel to the orbitomeatal line. The following early CT scan findings correlate with outcome1:

• Status of the basal cisterns.
• Midline shift.
• Subarachnoid hemorrhage in the basal cisterns.

Basal Cisterns at the Midbrain Level

Compressed or absent basal cisterns indicate a threefold risk of raised intracranial pressure and the status of the basal cisterns is related to outcome. The degree of mass effect is evaluated at the level of the midbrain. Cerebrospinal fluid cisterns around the midbrain are divided into three limbs, one posterior and two laterally (Fig. 1). Each limb can be assessed separately as to whether or not it is open or compressed. Basal cisterns can be:

• Open (all limbs open).
• Partially closed (one or two limbs obliterated).
• Completely closed (all limbs obliterated).

Midline Shift at the Foramen of Monro

The presence of midline shift is inversely related to prognosis. However, interaction exists with the presence of intracranial lesions and other CT parameters1. Midline shift at the level of the foramen of Monro should be determined by first measuring the width of the intracranial space to determine the midline ("A"). Next, the distance from the bone to the septum pellucidum is measured ("B") (Fig. 2). The midline shift can be determined by calculating:
Midline shift = (A/2) - B

Traumatic Subarachnoid Hemorrhage

Traumatic subarachnoid hemorrhage occurs in between 26 and 53% of all patients with severe traumatic brain injury. Mortality is increased twofold in the presence of traumatic subarachnoid hemorrhage. The presence of subarachnoid hemorrhage in the basal cisterns carries a positive predictive value of unfavorable outcome of approximately 70%.